Disk playback devices using pairs of endless drive belts to transfer disks between an eject position and a playback position have been disclosed in Japanese utility model publication no. 60-106250, Japanese utility model publication 61-24851, and Japanese laid-open publication no. 2-7263. In each of these devices, a pair of drive belts are held taut and parallel to each other on either side of a disk transfer path. A disk is supported between the belts by the disk's edges. With the disk supported between the belts, the distance separating the belts is approximately equal to the diameter of the disk. When the disk is transferred to the playback position, the drive belts are separated further apart to clear the disk for playback.
The belt used for disk transfer in the above-identified prior art devices must press against the rim of the disk to frictionally engage it. Elastic material is used in such belts to permit the belt to wrap snugly around support pulleys and to insure good frictional contact with the disk edge. However, the elasticity causes the long spanning portion between the pulleys to bow under the force of the disk edge. The bowing is, of course, greatest when the disk is in the center of the spanning portion.
The bowing of the drive belt makes less attractive designs for disk transfer devices employing a single belt and a fixed surface to support the disk edges instead of two revolving belts. While such a design would be simpler and more economical, the bowing of the drive belt would cause the path followed by the disk center, during disk transfer, to be a nonlinear function of angular displacement of the drive used to revolve the belt. This can make it difficult to arrange for the center of the disk to coincide precisely with a center of a turntable in a playback position. In addition, to design a single belt drive for a system capable of transporting different-sized disks would require consideration of a different non-linear function for each of the different disk sizes. While such design is theoretically possible, the variations in belt flexibility due to variations in material properties and resulting from manufacturing and use variables could make it impractical. In addition, complex arithmetic such as would be required requires more memory and processing power for a digital controller. Belt drive systems suffer from another problem in that vibration during disk transfer can cause a supported disk to be dislodges.